JP2006073579A - METHOD OF FORMING CRYSTALLINE ZINC OXIDE (ZnO) THIN FILM - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of forming a ZnO thin film having good crystallinity at a low temperature, at room temperature without heating it in particular. <P>SOLUTION: In the method of forming the crystalline zinc oxide thin film on a substrate at a temperature below 400°C, a crystalline buffer layer whose front surface is primarily a (111)-plane is formed on the substrate, and thereafter the zinc oxide thin film is deposited by a vapor phase method on the buffer layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for forming a crystalline zinc oxide (ZnO) thin film.

  Recently, zinc oxide (ZnO) has attracted attention because it has the same crystal structure and electronic structure as gallium nitride (GaN), which is a blue semiconductor laser material. In particular, since ZnO is an environmentally friendly oxide, it is expected as a thin film material for electronic devices such as a novel short wavelength light emitting material and a photovoltaic power generation material.

  Thin films for electronic devices may be required to have improved crystallinity in order to improve their characteristics. In particular, there is a demand for single crystallization of thin films for electronic devices by a vapor phase method in various applications. In addition, in order to enable application to a substrate or device having low heat resistance, it is also required to lower the temperature of the film formation process of the thin film for electronic devices. In particular, since the film can be formed at the room temperature level, the surface of the thin film is flattened at the nano level, and as a result, a steep interface can be obtained for electronic device applications, which is extremely useful.

  When a ZnO thin film is formed by a vapor phase method, when the thin film is made crystalline (including a polycrystal) and further made monocrystalline (epitaxially grown film), the thin film deposition substrate is usually set to 400 ° C. to Heat to a high temperature of 800 ° C. Such heat treatment is recognized in the art as being indispensable to promote atomic arrangement transfer for diffusion and crystallization of thin film materials on a substrate.

  However, such heat treatment at a high temperature can cause damage to the electronic device due to a difference in thermal expansion coefficient between the substrate and the thin film, interfacial thermal diffusion, or the like. In particular, when considering the combination of a thin film with low heat resistance such as an organic thin film and a crystalline ZnO thin film, it is an unavoidable problem to lower the temperature of the film forming process by the vapor phase method. It is also desired to improve the interface characteristics in electronic device applications by flattening the film surface of the crystalline ZnO thin film.

Transparent conductive film technology, Ohmsha, 1999, pages 135-144

  Accordingly, an object of the present invention is to provide a method for forming a ZnO thin film having good crystallinity at a low temperature, particularly at room temperature without heating.

  The above object is a method of forming a crystalline zinc oxide thin film on a substrate at a temperature of less than 400 ° C., and after providing a crystalline buffer layer whose surface is mainly a (111) lattice plane on the substrate. This is achieved by a method characterized by depositing a zinc oxide thin film on the buffer layer by a vapor phase method.

  According to the present invention, a ZnO thin film having good crystallinity, particularly in a single crystal form, can be formed at a low temperature, particularly at room temperature. Thus, since a crystalline ZnO thin film can be obtained at a low temperature, it can be easily combined with a thin film having low heat resistance such as an organic thin film, and the range of applications for electronic devices is widened. In addition, according to the present invention, since the flatness of the film surface of the crystalline ZnO thin film is improved, it is possible to improve the interface characteristics in electronic device applications.

  In the method according to the present invention, when a crystalline ZnO thin film is formed on a substrate at a temperature of less than 400 ° C., a crystalline buffer layer whose surface is mainly a (111) lattice plane is provided on the substrate, and then the buffer is formed. A ZnO thin film is deposited on the layer by a vapor phase method.

  When a ZnO thin film is formed by a vapor phase method, it is usually crystalline by heating the thin film deposition substrate to a high temperature of 400 ° C. to 800 ° C., and epitaxially grows to become single crystal in some cases. This is because heating promotes atomic arrangement movement for diffusion and crystallization of the deposited ZnO on the substrate. However, the present inventor has deposited an independent thin film that promotes atomic arrangement movement of ZnO, that is, a buffer layer on the substrate, and deposited ZnO on the substrate by vapor deposition, so that the present invention can be used at low temperatures and in some cases at room temperature. Thus, it was found that a ZnO thin film having excellent crystallinity can be obtained.

The buffer layer according to the present invention is a crystalline layer whose surface is mainly a (111) lattice plane. A person skilled in the art can understand a crystalline compound that grows on the (111) lattice plane. In particular, the buffer layer according to the present invention is preferably an inorganic compound thin film having a rock salt type crystal structure, a wurtzite type crystal structure or a fluorite type crystal structure. Those skilled in the art can also understand the crystal structure of rock salt type, wurtzite type or fluorite type. Specifically, as a thin film having a rock salt type crystal structure, a nickel oxide (NiO) thin film, a titanium nitride (TiN) thin film, etc., and as a thin film having a wurtzite type crystal structure, an aluminum nitride (AlN) thin film, gallium nitride. Examples of the thin film having a (GaN) thin film and the like having a fluorite-type crystal structure include a cerium oxide (CeO ₂ ) thin film. Further, as long as the surface becomes a (111) lattice plane, the buffer layer may be constituted by combining two or more layers.

  The buffer layer according to the present invention can be formed on the substrate by a vapor phase method. The substrate can be selected from ceramic, glass, plastic, silicon, metal, etc. according to the purpose. In particular, according to the present invention, since it can be processed at a low temperature of room temperature, a buffer layer can be formed on a substrate having low heat resistance or an organic material layer. The vapor phase method can be a conventional method. For example, sputtering methods such as magnetron sputtering and reactive sputtering, vapor deposition methods such as electron beam vapor deposition, laser ablation methods such as laser molecular beam epitaxy, chemical vapor deposition, and the like. A phase growth (CVD) method or the like is preferably used.

  The thickness of the buffer layer according to the present invention is generally in the range of 1 to 100 nm, preferably 2 to 20 nm. If the buffer layer is thinner than 1 nm, the objective of forming a crystalline ZnO thin film at a low temperature cannot be sufficiently achieved. On the contrary, when the thickness is larger than 100 nm, the unevenness of the buffer layer surface becomes large, and the flatness of the obtained ZnO thin film is lowered.

  According to the present invention, after providing the above crystalline buffer layer, a ZnO thin film is deposited thereon by a vapor phase method. The ZnO thin film thus deposited becomes crystalline, preferably single crystalline. When depositing the ZnO thin film, it is not necessary to heat the substrate carrying the buffer layer described above. Without heating, the deposition temperature is room temperature, ie approximately 20 ° C. ± 5 ° C. Of course, the substrate can be heated as long as the heat resistance of the substrate and other elements allows. Even in that case, it is not necessary to heat to a conventionally considered high temperature (400 ° C. or higher), and a crystalline, particularly single-crystal ZnO thin film can be formed at a low temperature of less than 400 ° C.

  According to the present invention, the zinc oxide can also include a dopant. By including a dopant in ZnO, the conductivity and other characteristics of the ZnO thin film can be improved. The method of including the dopant, that is, the doping method can be appropriately selected by those skilled in the art according to the gas phase method employed. For example, when the laser molecular beam epitaxy method is employed, a ZnO sintered body that contains a dopant in advance at a content corresponding to a desired thin film dopant concentration may be used as a target.

  Examples of the dopant for zinc oxide include aluminum (Al), indium (In), gallium (Ga), boron (B), silicon (Si), germanium (Ge), titanium (Ti), zirconium (Zr), and the like. It is done. The dopant concentration can be specifically determined in the range of 0.1 to 5% by mass, preferably 2 to 3% by mass, depending on the intended use.

  According to the present invention, a ZnO thin film containing or not containing a dopant is deposited by a vapor phase method on a crystalline buffer layer whose surface is mainly a (111) lattice plane. As the vapor phase method, the conventional method described above for the buffer layer may be appropriately employed depending on the purpose. The deposition rate by the vapor phase method is generally 10 μm / hour or less, preferably 0.1 μm / hour or less. In general, the slower the deposition rate, the better the crystallinity of the deposited film. However, when considering industrial productivity, a rate of about 10 μm / hour may be required. A person skilled in the art can appropriately adjust and control the deposition rate by the vapor phase method according to the employed specific method.

  According to the present invention, the deposited zinc oxide exhibits an orientation that grows on the (0001) lattice plane. It was found that the crystallinity of the ZnO thin film grown at such a low temperature, particularly at room temperature, is no different from the crystallinity of the ZnO thin film grown at a high temperature by the conventional method, and in particular, it is + c polarity growth with excellent optical characteristics. . Such a technique of forming a single crystal ZnO thin film having + c polarity at room temperature is an effective means for inexpensive production at the time of mass production of light emitting devices. In addition, it is advantageous in that damage due to high-temperature heating is reduced even when combined with an organic thin film or the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples.

  For the formation of the buffer layer and the ZnO thin film, a laser molecular beam epitaxy (laser MBE) apparatus (Lambda Physic, Model: LPX-100) equipped with a KrF excimer laser with a wavelength of 248 nm was used. In the evaluation of the crystallinity of the obtained thin film, reflection high-energy electron diffraction (RHEED) and X-ray diffraction were used. Atomic force micrographs were used to evaluate the flatness of the obtained thin film. For the evaluation of the surface structure (polarity) of the obtained thin film, coaxial type direct collision ion scattering spectroscopy (CAICISS) was used.

Comparative Example 1
A ZnO film having the thickness shown in Table 1 was directly formed on a very flat sapphire (single crystal alumina) (0001) substrate at room temperature (about 20 ° C.) using the laser MBE apparatus. A ZnO sintered body was used as a target. The laser energy density, laser frequency, and atmospheric pressure on the target were set as shown in Table 1 below.

  The RHEED pattern from two directions (<1120>, <1010>) of the obtained ZnO thin film is shown in FIG. The RHEED patterns from the two directions were the same, and a different pattern peculiar to the single crystal film was not obtained. Even if the direction is changed in this way, the RHEED pattern does not change, which means that the crystal direction in the substrate surface is random. Therefore, it can be seen from the obtained RHEED pattern that the thin film was a polycrystalline alignment film that was randomly oriented in the substrate plane and oriented only in the direction perpendicular to the substrate. Moreover, the X-ray-diffraction measurement result of the obtained ZnO thin film is shown in FIG. From this result, it can be seen that the thin film was a polycrystalline alignment film oriented in the (0001) direction.

  FIG. 3 shows an atomic force microscope surface image of the obtained ZnO thin film. In the figure, RMS is an index indicating the surface roughness (mean square roughness). From this result, it can be seen that the thin film was a very rough film having a surface roughness of 0.77 nm. Moreover, since the surface structure (polarity) of the obtained thin film was a polycrystalline alignment film, it was confirmed by CAICISS measurement that it was a film in which + c polarity and −c polarity were mixed.

Examples 1-4
(A) TiN (Example 1), (b) on a very flat sapphire (single crystal alumina) (0001) substrate as a buffer layer at room temperature (about 20 ° C.) using the laser MBE apparatus. NiO (Example 2), (c) AlN / TiN (Example 3) and (d) AlN / NiO (Example 4) were deposited. The laser energy density, laser frequency, and atmospheric pressure on each target were set as shown in Table 1. Further, Table 1 shows the thickness and surface orientation of each buffer layer. Here, the description of AlN / TiN and AlN / NiO indicates that TiN or NiO was first deposited on the substrate and then AlN was deposited thereon.

  On each of the obtained buffer layers, a ZnO film having a thickness shown in Table 1 was formed at room temperature (about 20 ° C.) using the laser MBE apparatus. A ZnO sintered body was used as a target. The laser energy density, laser frequency, and atmospheric pressure on the target were set as shown in Table 1.

  The RHEED pattern of each obtained ZnO thin film is shown in FIG. For any of the thin films (a) to (d), a linear (streaky) RHEED pattern is obtained, and the RHEED patterns in two directions (<1120> and <1010>) are different. This indicates that the crystal orientations are aligned in the substrate plane, and these thin films have grown single crystal (epitaxially). Moreover, the X-ray-diffraction measurement result of each obtained ZnO thin film is shown in FIG. In any of the thin films (a) to (d), a (0002) diffraction peak peculiar to ZnO was confirmed, which is stronger than the peak of Comparative Example 1 for reference. That is, it can be seen that the thin film was an epitaxial thin film having (0001) orientation due to the buffer layer effect and having a uniform crystal orientation in the substrate plane.

  An atomic force microscope surface image of each obtained ZnO thin film is shown in FIG. In the figure, RMS is an index indicating the surface roughness (mean square roughness). From these surface images, it was found that any of the thin films (a) to (d) grew while leaving the atomic step structure of the sapphire substrate, and it was confirmed that these surfaces were very flat. . Moreover, it was confirmed from the CAICISS measurement that the surface structure (polarity) of the obtained thin film has + c polarity.

Example 5
On the silicon (Si) (111) substrate, CeO ₂ (thickness 3 nm) was formed as a buffer layer at room temperature (about 20 ° C.) using the laser MBE apparatus. The laser energy density, laser frequency and atmospheric pressure on the CeO ₂ target were set as shown in Table 1. The surface orientation of the buffer layer was (111).

  On the obtained buffer layer, a ZnO: Al film having a thickness of 100 nm doped with 0.3% by mass of aluminum was formed at room temperature (about 20 ° C.) using the laser MBE apparatus. A ZnO sintered body doped with 0.3% by mass of Al was used as a target. The laser energy density, laser frequency, and atmospheric pressure on the target were set as shown in Table 1.

  The RHEED pattern of the obtained ZnO: Al thin film is shown in FIG. A linear (streaky) RHEED pattern is obtained, and the RHEED patterns in two directions (<1120> and <1010>) are different. This indicates that the crystal orientations are aligned in the substrate plane, and these thin films have grown single crystal (epitaxially). Moreover, the X-ray-diffraction measurement result of the obtained ZnO: Al thin film is shown in FIG. A (0002) diffraction peak peculiar to ZnO was confirmed, that is, it was found that the thin film was an epitaxial thin film having a (0001) orientation due to the buffer layer effect and having a uniform crystal orientation in the substrate plane.

5 is a drawing-substituting photograph showing a reflection high-energy electron diffraction (RHEED) pattern (crystal structure) of the ZnO thin film obtained in Comparative Example 1. FIG. 4 is a graph showing an X-ray diffraction pattern of a ZnO thin film obtained in Comparative Example 1. 5 is a drawing-substituting photograph by an atomic force microscope showing a surface image (thin film surface texture) of a ZnO thin film obtained in Comparative Example 1. FIG. It is a drawing substitute photograph which shows the reflection high-energy electron diffraction (RHEED) pattern (crystal structure) of each ZnO thin film obtained in Examples 1-4. It is a graph which shows the X-ray-diffraction pattern of each ZnO thin film obtained in Examples 1-4. It is a drawing substitute photograph by the atomic force microscope which shows the surface image (thin film surface structure | tissue) of each ZnO thin film obtained in Examples 1-4. 6 is a drawing-substituting photograph showing a reflection high-energy electron diffraction (RHEED) pattern (crystal structure) of a ZnO: Al thin film obtained in Example 5. FIG. 6 is a graph showing an X-ray diffraction pattern of a ZnO: Al thin film obtained in Example 5. FIG.

Claims (9)

  A method of forming a crystalline zinc oxide thin film on a substrate at a temperature of less than 400 ° C., wherein after providing a crystalline buffer layer whose surface is mainly a (111) lattice plane on the substrate, A method comprising depositing a zinc oxide thin film thereon by a vapor phase method.   The method according to claim 1, wherein the buffer layer is an inorganic compound thin film having a rock salt type crystal structure, a wurtzite type crystal structure or a fluorite type crystal structure. The method of claim 1, wherein the buffer layer is a NiO thin film, a TiN thin film, an AlN thin film, a GaN thin film, a CeO ₂ thin film, or a combination thereof.   The method according to claim 1, wherein the thickness of the buffer layer is in the range of 2 to 20 nm.   The method according to any one of claims 1 to 4, wherein the zinc oxide thin film is monocrystalline.   The method according to claim 1, wherein the zinc oxide thin film contains a dopant.   The method of claim 6, wherein the dopant is aluminum.   The method according to claim 1, wherein the temperature is 20 ° C. ± 5 ° C.   The method according to any one of claims 1 to 8, wherein the gas phase method is performed by a laser molecular beam epitaxy method.